1. Field of the Invention
The present invention relates to a contact charging member fitted to an image forming apparatus such as an electrostatic copying machine. It also relates to an apparatus making use of it.
2. Related Background Art
The electrophotographic copying apparatus comprises a means for uniformly charging a charge-receiving member to a given potential. As a means for the charging, almost all of apparatus having been put into practical use utilize the corona charging method. This, however, has the problems that it requires a high voltage and also generates ozone in a large quantity.
Under such circumstances, recently proposed is a contact charging method in which a voltage is externally applied to a charging member and this charging member is brought into contact with a charge-receiving member.
According to this contact charging method, for example, a charging roller serving as a contact charging member is rotated in contact with an electrophotographic photosensitive drum serving as the charge-receiving member, where a V.sub.AC +V.sub.DC voltage obtained by superimposing an alternating voltage V.sub.AC on a direct voltage V.sub.DC is applied to the charging roller, so that the photosensitive drum can be uniformly charged.
Hitherto, an elastic surface conductive layer, of a contact charging member such as a charging roller, is formed by dispersing a conductive pigment in a polymeric elastic material. However, it is difficult to obtain the uniform dispersibility of the conductive pigment not only for each contact charging member manufactured, but within a conductive layer of a contact charging member, resulting in an uneven resistance. This resistance unevenness brings about the following problems. For example, although the charging roller must be kept conductive, when a defect such as a pinhole has occurred in a photosensitive layer of a photosensitive drum comprising a conductive substrate and a photosensitive layer formed thereon and the resistance of the charging roller is excessively low, a large electriccurrent may flow, because of a leak, from the low-resistance part of the contact charging member into the pinhole area, lowering the output of a high voltage power supply, so that the photosensitive drum is poorly charged at the area of pinhole. On the other hand, at the area of high-resistance, the surface of the photosensitive drum can not be uniformly charged resulting in spot-like charge unevenness which, by an ordinary electrophotographic image forming process, gives an output image having black spots corresponding to the charge unevenness, so that high quality image can not be obtained.